You apparently have the option of removing a Group access from a document if you are a member of that Group, even though the document's parent is a member of that Group.

You can't actually do this, it just appears that you can as the Group box is not grayed-out. You are allowed to untick this box, as though you've unselected a Group, and then submit the amendment by saving the page. But then if you go back to the Group box in question, you find it still ticked and now grayed-out. This is how it should have appeared in the first place.